In the pulp and paper industry it common to use rotating vacuum filter assemblies, arranged in series of axially spaced disks, to dewater and/or thicken pulp stock chests, to recover fibre and to clarify white water for re-use in applications such as shower water for filters. The general operation and configuration of vacuum filters are further described in U.S. Pat. No. 3,331,512 which issued Jun. 18, 1967 (VORE) and in U.S. Pat. No. 3,283,906 which issued Nov. 8, 1966 (CRANE).
A filter element, as used herein, refers to one of a series of wedge-shaped filtering units that are mounted to and extend radially from a hollow central hub to form a coaxial circular disk-like filter assembly. The number of filter elements that make up each circular filter assembly typically vary between eight and twenty. The central hub rotates about an axis positioned so that the filter assembly is partially submerged in a slurry bath.
Each filter element comprises two oppositely facing filter surfaces covered by a suitable filter media such as polyester or stainless steel mesh. The filter media rests upon the outermost parts of the filter element's filter surface which is typically a plate material which has been perforated or punched to permit passage of the filtrate, as illustrated at 114 in FIG. 4 of CRANE. Each filter element defines one or more central chambers. These chambers communicate, through the necked end of the filter element, with the hollow core of the central hub.
As the filter elements rotate through the lower portion of their circular path they are submersed into a slurry of pulp fibres suspended in liquid. A vacuum source, connected to the hollow central hub, is used to urge liquid to pass through the filter media, into the central chambers of the filter element and then into the central hub. The outer surface of the filter media retains the pulp fibre resulting in the formation of a fibre mat on each of the opposed filter surfaces.
Immediately upon emergence of the filter element from the slurry bath the vacuum source is interrupted. The fibre mat is further dewatered by residual vacuum until the filter element reaches a point in its circular path where a knock-off liquid shower or where scrapers are used to detach the fibre mat from the filter media.
The mat peels off and falls under its own weight into chutes placed adjacent to the surface of the filter assembly. Fixed or oscillating showers are then used to clean any remaining fibre off the filter element prior to its re-submersion in the liquid-fibre bath, to start the cycle over again.
Filter elements are typically removably mounted to the central hub to provide for regular maintenance and replacement. Their design and dimensions vary with each manufacturer of vacuum filter equipment. The replacement cost of each filter element, their efficiency of operation, their ease of manufacture and replacement, their weight and their durability are important factors to operators as they impact operating costs and equipment down-time. The efficiency of a filter element ultimately depends on the filter surface area that is operable at any one time. Weight reductions in the filter element reduce operating power consumption and ease task of filter element replacement.
Conventional vacuum disk filters suffer from inefficiencies caused by obstructed filter surface areas. This may be due to either the structure of the filter-media-supporting perforated plate itself, or due to the tendency of fibre to build-up between the filter media and this underlying plate. Fibre build-up causes a reduction in the operable filter area and creates localized pressure zones in remaining unblocked areas. These pressure zones may ultimately rupture the filter media.
Efforts have been made to increase the effective filtration surface area of conventional filter elements by increasing the size and/or number of holes in the perforated media-supporting plate surface. However, this tends to weaken the overall filter element structure and predisposes the perforated plate to stress cracking. Too few perforations restricts filtrate flow and creates a larger surface area for the build-up of fibre between the filter media and the perforated plate.
Thus, a lightweight filter element that maximizes filter area, minimizes the tendency for fibre build-up, and that may be retrofit to existing vacuum filter equipment in a cost effective manner would be desirable.